Fuse cells are widely used in ICs in order to make the ICs tunable. For example, after an IC designed by an IC supplier is manufactured it may happen that, due to tolerance in the manufacturing process, the performance of the ICs is not what was intended. In this case, the performance of the ICs can be modified by cutting a selection of the fuses before the ICs are supplied to customers. As an example, fuse cells can be used to store addressing information of defective memory cells in an array for redundancy applications.
When the IC supplier contemplates cutting the fuses of an IC it may wish to check that the resulting performance of the IC will be what is desired. For that reason, it is known to provide circuitry on the IC for simulating the cut and uncut fuse states and which is controllable using control signals. Control signals are applied to the circuit to cause this circuitry to simulate the proposed cutting of fuses, and the performance of the IC is then investigated.
FIG. 1 shows a conventional fuse cell 101. The fuse cell typically includes a fuse 110 coupled between a pull-up circuit 105 (at a voltage (e.g. a positive voltage) which may represent a logic 1) and a pull-down circuit 106 (at a voltage (e.g. ground) which may represent a logic 0).
Depending on the state of the fuse (cut or uncut), the fuse cell 101 generates a fuse cell output signal at a fuse cell output terminal 160 which is commonly coupled to the fuse 110 and pull-up power source 105. As illustrated, the pull-down circuit 106 is decoupled from the power-up circuit 105 when the fuse is cut, producing a logic 1 fuse cell output signal at terminal 160. On the other hand, an uncut fuse couples the output terminal 160 to the pull-down circuit 106, thus generating a logic 0 output signal.
If the fuse is uncut, a current path exists between the power-up and power-down circuits 105, 106. As a result, power dissipates from the power-up circuit 105 to ground 106 when the fuse is uncut. This leads to an increase in power consumption, particularly since one of the design goals is to minimize the need to cut fuses in the IC. For low power or portable applications, particularly, the increased power consumption is undesirable and, in some cases, unacceptable.
As evidenced from the above discussion, it is desirable to provide an improved fuse cell with reduced or no static power dissipation.
The invention relates generally to fuse cells. In particular, the invention relates to software programmable fuses having reduced or no static power consumption.
In general terms, the present invention proposes that a fuse cell utilizes a latch for storing the state of the fuse. In a first operating mode, the latch is coupled to the fuse, and outputs a signal which depends on the state of the latch. In a second operating mode, the fuse cells are arranged to generate an output signal which is independent of the fuse state and instead determined by a software programmable fuse circuit under the control of control signals. In the first operating mode, the use of the latch avoids having a pull-up power source coupled to ground when the fuse is uncut as with conventional fuse cells.
In a first form of the invention, the fuse cell includes a control circuit, a fuse circuit, a software programmable fuse circuit, and a latch. The control circuit is coupled to the latch, fuse circuit and software fuse circuit. In response to fuse cell input signals, the control circuit causes the fuse cell to operate in either a first or second operating mode. In the first operating mode, the control circuit couples the latch to the fuse circuit, enabling the latch to store the fuse state. In the second operating mode, the control circuit puts the latch into a first logic state or a second logic state depending on the fuse state to be simulated.
In a second form of the invention, the fuse cell also includes a control circuit, a fuse circuit, a software programmable fuse circuit, and a latch. However, in this case the control circuit need only be coupled to the latch and the fuse circuit. The software programmable fuse circuit is connected to the output of the latch, and in response to fuse cell input signals outputs either a signal depending on the signal received from the latch or alternatively a signal determined by the fuse cell input signals. Note that it is not necessary that all components of the fuse cell are provided in the same block of the circuitry. Instead, it may be convenient to provide at least one fuse block including the control circuits, fuse circuits and latches of one or more of the fuse cells, and at least one softfuse block including the software programmable fuse circuit.
In either case, preferably, the control circuit can also be operated in a initialization mode. The control circuit further comprises an initialization circuit. During power-up, for example, an active initialization signal is generated to put the control circuit into the initialization mode. The active initialization signal couples the initialization circuit to the latch to initialize the latch to a first known state. After the latch has been initialized, the initialization signal becomes inactive and the fuse cell operates in either the first or second operating mode.